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Q. I understand some head lice products contain active excipients or can be used at higher strengths. What should I know about this in terms of effectiveness, and how do they work?

A. Some head louse preparations contain excipients that either increase the effectiveness of the insecticide or help overcome problems caused by resistance. Alcoholic malathion lotions contain terpenoids (components of essential oils) that were originally incorporated as fragrances.

Tests of essential oils against other insects show these chemicals can kill insects and help to penetrate insect eggs.

During the 1990s, when alcoholic malathion products were formulated without terpenoids, the products were less effective (Burgess, 1991).

Similarly, some solvents can assist insecticide activity. When aqueous carbaryl

was formulated the product included

two solvents that helped hold the insecticide in the emulsion and coincidentally appeared to facilitate penetration into lice and eggs.

That product is now rarely used but the same vehicle was used for aqueous 0.5% phenothrin liquid.

The effect of the solvents combined with the 12-hour application time for the

product have shown it to be more effective than other products containing the same insecticide, achieving more than 70 per cent cure in a recent clinical study (Burgess et al, 2005).

Increasing application time effectively increases the dose applied to the lice but is not suitable for many formulations. Using an alternative, eg 5% permethrin cream in place of 1% permethrin creme rinse, an approach used by some US physicians, may improve performance.

But if lice exhibit resistance this is only likely to be a short-term option, whereas the effects produced by excipients are not affected by resistance to insecticides, unless the insects acquire a specific tolerance of those chemicals also.

Ian Burgess, consultant medical entomologist, Insect Research and Development Ltd, Royston, and consultant author on the anti-infective skin preparations section of the BNF

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